JP2005339654A - Optical head device - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an optical head device which allows recording and reproducing speeds to be increased without enlarging the device even when diffraction efficiency, especially, a diffraction rate of 1st-order diffracted light in one side is improved to shorten a wavelength of a light source. <P>SOLUTION: A polymer dispersed liquid crystal type polarization selective hologram element 100 forms a periodical phase separation structure of a layer mainly consisting of a polymer and a layer mainly consisting of a non-polymerizable liquid crystal by holding a composition comprising at least: a non-polymerizable liquid crystal 102 having dielectric anisotropy; a polymerizable monomer 103 or a prepolymer; and a photopolymerization initiator between a pair of transparent substrates 101 and by subjecting this composition to two-beam interference exposure. The hologram element 100 is used as a hologram element for use in an optical head, and a direction of polarization of light impinging on the hologram element 100 from the light source (an oscillation direction of an electric field) is set approximately in parallel with a recording medium track direction. <P>COPYRIGHT: (C)2006,JPO&NCIPI

Description

  The present invention relates to an optical head device, and more particularly to an optical pickup and a hologram element in an optical disc drive device used when recording / reproducing a plurality of standard optical discs having different wavelengths to be used, such as CD, DVD, DVD + Blue. .

  As is well known, one of optical elements that use laser light as a light source is an optical head that is used in an optical disk drive device, and one configuration that is used in an optical head is a configuration that uses a polarization separation element (for example, Patent Document 1).

In the above-mentioned Patent Document 1, a medium having birefringence (optical anisotropy) having a rectangular uneven shape is disposed on a transparent substrate, and an optically isotropic medium is filled on the medium. A configuration using a polarizing diffraction grating covered with a transparent substrate is shown.
In this configuration, by making the refractive index of the isotropic medium equal to either the ordinary refractive index or the extraordinary refractive index of the birefringent medium, a diffraction grating exhibiting polarization (optical anisotropy) can be obtained. This polarization separation element is arranged in the optical path so as to have functions of forward transmission and backward diffraction.

JP-A-10-302291 (paragraph "0042" column)

  FIG. 15 is a schematic diagram of a conventional optical head device. When the irradiation light from the semiconductor laser 8 serving as the light source in FIG. 15 passes through the polarizing diffraction grating 7 whose structure is shown in FIG. Then, the light is reflected on the recording surface of the optical recording medium 13 via the quarter-wave plate 11 and the objective lens 12, and the reflected light is polarized after the vibration direction is converted by the quarter-wave plate into a direction orthogonal to the outgoing direction. Diffracted light is detected by the diffractive diffraction grating 7 and detected by the photodetector 9.

FIG. 16 is a diagram showing a configuration of the polarizing diffraction grating 7, in which the polarizing diffraction grating 7 is a medium 2 having birefringence (optical anisotropy) having a rectangular uneven shape on the transparent substrate 1. Is disposed, filled with an optically isotropic medium 3, and covered with a transparent substrate 1 '.
By making the refractive index of the isotropic medium 3 equal to either the ordinary refractive index or the extraordinary refractive index of the birefringent medium 2, the isotropic medium 3 functions as a diffraction grating exhibiting polarization (optical anisotropy). That is, it is possible to provide a characteristic that almost completely transmits a polarized light in a certain direction and totally diffracts a polarized light orthogonal thereto.
If such a polarizing diffraction grating is used as a branching element of an optical head, the outgoing path from the light source to the optical recording medium 13 is set to a polarization direction that totally transmits, and is efficiently condensed on the optical recording medium, and 1 in the optical path. By arranging the / 4 wavelength plate, the reflected light from the optical recording medium 13 returns to the polarization diffraction grating 7 again so as to return perpendicularly to the polarization direction of the forward path, as shown in FIG. As a result, the return path light is totally diffracted so that it can be efficiently received by the photodetector 9, and a high-efficiency optical head can be realized for both the forward and return paths.

  FIG. 17 is a diagram showing the diffraction efficiency characteristics of the + 1st order diffracted light (one-side diffracted light) with respect to the incident angle of the polarizing diffraction grating shown in FIG. 16. As a characteristic of the rectangular grating, about 40% centering on vertical incidence. With a diffraction efficiency of

  A conventional optical head was sufficient with a polarizing diffraction grating having a diffraction efficiency as shown in FIG. 17, but the recording / reproducing speed (particularly the reproducing speed) of the optical disk apparatus on which the optical head shown in FIG. When trying to increase the speed, there were the following problems.

In other words, when increasing the recording / reproducing speed, the polarizing diffraction grating needs to have a diffraction efficiency of 40% or more in order to improve the SN ratio when the reflected signal from the disk is received by the photodetector. come. In particular, when applied to high-density optical discs that use a semiconductor laser in the blue region as a light source, the reproduction signal band becomes wider due to the higher recording information density, and at the same time, the sensitivity of the photodetector is lower in the blue region than in the red or infrared region. The SN ratio of the photodetection signal is reduced due to two aspects of doing so.
In order to improve this SN ratio reduction, the return diffraction efficiency (+ 1st-order diffraction efficiency) of the polarizing diffraction grating requires a high diffraction efficiency exceeding 40%. Further, in order to reduce the size by integrating the light source and the light detector in the blue region, that is, in order to make the polarizing diffraction grating 7, the semiconductor laser 7 or the light detector 9 in FIG. As the required grating pitch becomes shorter, a narrow pitch of 1 μm order is required.

  In consideration of the structural requirements as a polarizing diffraction grating with a narrow pitch and high diffraction efficiency, which will be required in response to the future shortening of the wavelength of the light source, the rectangular grating shown in FIG. The blazed grating, which is a method for increasing the efficiency of the + 1st order diffraction efficiency on one side compared to the conventional one, can achieve a high diffraction efficiency of 80 to 90%, but it is 1 μm. Narrowing the order is difficult to implement because of the high degree of difficulty in processing.

  In view of the above problems in the conventional optical head, it is an object of the present invention to increase the diffraction efficiency, particularly the diffraction rate of the first-order diffracted light on one side and reduce the wavelength of the light source without increasing the size. An object of the present invention is to provide an optical head device that can increase the speed of the optical head device.

  The invention according to claim 1 is an optical head device including an optical head for condensing light from a light source on a recording medium, detecting reflected light from the recording medium, and recording or reproducing information, or recording and reproducing information. As a hologram element used for the optical head, at least a composition comprising a non-polymerizable liquid crystal having dielectric anisotropy, a polymerizable monomer or prepolymer, and a photopolymerization initiator is held between a pair of transparent substrates, By subjecting the composition to two-beam interference exposure, a polymer-dispersed liquid crystal type polarization selective hologram element in which a periodic phase separation structure of a layer mainly composed of a polymer and a layer mainly composed of a non-polymerizable liquid crystal is formed. And the polarization direction of light incident on the hologram element from the light source (electric field vibration direction) is set substantially parallel to the recording medium track direction.

  According to a second aspect of the present invention, in the optical head device according to the first aspect, the focus detection diffracted by the hologram as the polarization direction of the return light reflected by the recording medium and incident on the hologram element (vibration direction of the electric field) The optical axis of the diffracted light is set substantially parallel to the direction projected onto the hologram surface.

  According to a third aspect of the present invention, in the optical head device according to the first or second aspect, the hologram element further includes a polymerizable liquid crystal monomer in the composition constituting the hologram, and mainly after two-beam interference exposure. A polarization-selective hologram element containing a liquid crystal skeleton in a polymer layer is used.

  According to a fourth aspect of the present invention, in the optical head device according to any one of the first to third aspects, a polymer-dispersed liquid crystal type polarization selective hologram element is used as the hologram element, and a semiconductor laser is used as a light source. The semiconductor laser is characterized in that the bonding direction of the semiconductor laser (the bonding direction of the active layer) is set in a direction perpendicular to the track direction of the recording medium, and a half-wave plate is installed between the light source and the hologram element.

  According to a fifth aspect of the invention, in the optical head of the fourth aspect, the half-wave plate is integrated with a hologram element.

  A sixth aspect of the present invention is the optical head device according to any one of the first to fifth aspects, wherein the light source, the photodetector, and the hologram element are integrated.

  According to the first aspect of the present invention, there is provided a polymer-dispersed liquid crystal type polarization selective hologram in which a periodic phase separation structure of a layer mainly composed of a polymer and a layer composed of a non-polymerizable liquid crystal is formed by two-beam interference exposure. Since the polarization direction of light incident on the hologram element from the light source (the vibration direction of the electric field) is set substantially parallel to the recording medium track direction, the forward path from the light source to the optical recording medium can be achieved with a grating pitch of 1 μm or less. Since the hologram element is not diffracted and is incident on the optical recording medium with high efficiency, the recording speed can be increased.

  According to the second aspect of the present invention, the polarization direction of the return light reflected by the recording medium and incident on the hologram element (vibration direction of the electric field) is the optical axis of the diffracted light for focus detection diffracted by the hologram. Is set substantially parallel to the direction projected onto the hologram surface, the + 1st-order diffraction efficiency on one side can be made high efficiency of 80% or more, and the amount of light received by the photodetector can be increased.

  As a result, the S / N ratio of the detection signal can be increased and the reproduction speed can be increased. Further, since a pitch of 1 μm or less is possible, the distance between the hologram element and the photodetector can be reduced, and the optical head can be miniaturized.

According to the third aspect of the present invention, the polarization selection further includes a polymerizable liquid crystal monomer in the composition constituting the hologram, and a liquid crystal skeleton portion in the layer mainly composed of the polymer after the two-beam interference exposure. When a two-beam interference exposure is performed, the birefringence of the non-polymerizable liquid crystal part increases using the two-beam interference exposure and the diffraction polarization is increased. The selectivity can be increased.
In addition, since the interaction between the polymer wall and the non-polymerizable liquid crystal molecules is strong, the non-polymerizable liquid crystal part can maintain the liquid crystal phase up to a relatively high temperature region, and can widen the operating temperature range of the polarization selective hologram element. It becomes possible.

  According to the fourth aspect of the present invention, a semiconductor laser is used as the light source, the bonding direction of the semiconductor laser (the bonding direction of the active layer) is set in a direction perpendicular to the track direction of the recording medium, and the gap between the light source and the hologram element is set. Since a half-wave plate is installed, for high-density optical discs, both reduction in reproduction jitter by improving the reading resolution of recording marks and improvement in the efficiency of light utilization from the light source to the optical recording medium are achieved. Thus, it is possible to provide an optical head capable of high-speed recording with low jitter.

  According to the fifth aspect of the present invention, the half-wave plate disposed between the light source and the hologram element is integrated with the hologram element, thereby reducing the number of parts and simplifying by omitting the half-wave plate holder member. And cost reduction.

  According to the invention described in claim 6, by integrating the light source, the photodetector, and the hologram element, the optical head can be miniaturized, the optical head assembly adjustment can be simplified, and the temperature change and the characteristics over time can be achieved by integration. Therefore, it is possible to provide a stable optical head.

  The best mode for carrying out the present invention will be described below with reference to embodiments shown in the drawings.

FIG. 1 shows a polarization-selective hologram element using a liquid crystal material used in an optical head apparatus which is an embodiment according to the first aspect of the present invention. A method for producing the element will be described below.
In FIG. 1, a polarization selective hologram 100 is composed of a composition in which a non-polymerizable liquid crystal molecule 102 and a polymerizable monomer 103 or a prepolymer are uniformly mixed with a photopolymerization initiator (not shown). The transparent substrates 101 and 101 ′ are sandwiched.
The thickness of the composition can be controlled by a spacer member (not shown) that controls the distance between the substrates. Since this composition has photosensitivity, it is preferable to handle it in an environment in which light having a sensitive wavelength region is blocked in the device manufacturing process.

As the non-polymerizable liquid crystal 102, a general liquid crystal having refractive index anisotropy can be used.
When selecting a liquid crystal material, a liquid crystal material having a refractive index equal to the refractive index of the cured layer of the polymerizable monomer 103 or the prepolymer may be selected in an orientation state with a certain order parameter, or a liquid crystal material may be selected. Then, the polymerizable monomer or the prepolymer may be selected so that the refractive index is the same as the refractive index of the liquid crystal in a certain order parameter alignment state.
Further, as the polymerizable monomer 103 or its prepolymer, it is preferable to use a monomer that has a large cure shrinkage due to polymerization.

  As the photopolymerization initiator, a known material can be used. When the amount of the photopolymerization initiator added is too small, phase separation between the polymer and the liquid crystal is difficult to occur, and the necessary exposure time is increased. On the other hand, if there are too many photopolymerization initiators, the polymer and liquid crystal are cured with insufficient phase separation, so that many liquid crystal molecules are taken into the polymer and the polarization selectivity is reduced. There is.

As the spacer member, it is possible to use a spherical spacer, a fiber spacer, a film or the like as used in a liquid crystal display device. Further, the protrusion shape may be processed on the substrate surface by photolithography and etching or molding technique. The spacer member is preferably formed outside the effective area of the hologram. The height of the spacer member is preferably in the range of several μm to several tens of μm, and is appropriately set so as to have a desired hologram layer thickness according to the wavelength of the diffracted light and the refractive index difference between the polymer portion and the liquid crystal portion.
As the transparent substrates 101 and 101 ′, glass, plastic, or the like used in a liquid crystal display device can be used.

Next, a hologram forming process by phase separation will be described with reference to FIGS.
When exposure is performed on the composition using a two-beam interference exposure system using a laser light source having a desired wavelength (not shown in FIG. 2), as shown in FIG. (Indicated by triangles) or the prepolymer photopolymerization reaction begins. At this time, curing shrinkage occurs, resulting in a density difference, the adjacent polymerizable monomer or prepolymer moves to the bright part, and further polymerization proceeds. At the same time, phase separation occurs when non-polymerizable liquid crystal (shown by an ellipse for convenience) existing in the bright part is driven out toward the dark part. At this time, it is considered that when the liquid crystal molecules move, a force for aligning the major axis of the liquid crystal molecules in the moving direction by the interaction with the monomer or polymer chain is considered to work. That is, it is considered that a force for orienting liquid crystal molecules in the direction of the interference fringe acts in the phase separation process.

Finally, as shown in FIG. 4, a periodic structure of the polymer layer and the non-polymerizable liquid crystal layer is formed corresponding to the bright and dark pitches of the interference fringes, and the orientation vector of the liquid crystal layer portion is oriented in the interval direction of the interference fringes. It is thought that the state that was In this interference exposure and phase separation process, the sample is preferably heated and held at an appropriate temperature.
It is considered that the phase separation speed changes depending on the temperature and affects the orientation of the liquid crystal molecules. The optimum heating temperature varies depending on the material used, but is preferably about 40 ° C to 100 ° C.

In this embodiment, by appropriately setting the combination of the liquid crystal type and the polymer type so that the ordinary light refractive index no of the entire liquid crystal part and the refractive index np of the polymer part substantially coincide with each other, Is not diffracted because it does not feel the difference between the ordinary light refractive index no of the whole liquid crystal part and the refractive index np of the polymer part, and for the p-polarized incident light, the extraordinary light refractive index ne of the whole liquid crystal part and the refractive difference of the polymer part A polarization-selective hologram that feels and diffracts is obtained. Here, since the diffraction efficiency of the volume hologram depends on the product Δn · d of the refractive index modulation amount Δn and the thickness d, the hologram thickness d can be reduced if the refractive index difference Δn can be increased.
If the thickness of the volume hologram is reduced, the angle dependency of the diffraction efficiency is reduced, and the decrease in light utilization efficiency against the incident angle fluctuation is improved.
Therefore, as shown in FIG. 4, a highly efficient polarization selective hologram having a large polarization selectivity and a relatively small incident angle dependency can be obtained.

The hologram element interference-recorded on the recording material containing liquid crystal molecules described in FIGS. 2 to 4 is used for the optical head having the configuration shown in FIG.
FIG. 5 shows a semiconductor laser as a light source of the optical head (for convenience, reference numeral 8 shown in FIG. 15) LD, a photodetector (for convenience, reference numeral 9 shown in FIG. 15) PD, and a portion of the hologram element 100 Is an enlarged version. Therefore, in FIG. 8, the collimating lens, the objective lens, the optical recording medium, and the like are not shown.
A hologram element 100 subjected to interference exposure on the recording material 13 containing liquid crystal molecules described in FIGS. 2 to 4 in the divergent light emitted from the semiconductor laser 8 is disposed.
In the hologram element 100, when the return light reflected by the optical recording medium is diffracted by the hologram element 100 and received by the photodetector 9, the optical axis of the diffracted light for detecting the focus signal in the diffracted light is shown in FIG. It is set in the yz plane.
At this time, the direction of the track for recording information on the optical recording medium for recording / reproducing information is the direction perpendicular to the paper surface in FIG.

  The embodiment of the invention according to the first aspect is characterized in that the polarization direction (electric field) of light incident from the light source side 8 on the hologram element produced by interference exposure on the recording material containing liquid crystal molecules described in FIGS. (Vibration direction) is set parallel to the track direction.

  By setting the polarization direction of light incident from the light source 8 side in parallel with the track direction of the optical recording medium, the light source for the hologram element 100 created by interference exposure on the recording material containing the liquid crystal molecules described in FIGS. Since the hologram element 100 does not feel the difference between the ordinary light refractive index no of the entire liquid crystal part and the refractive index np of the polymer part with respect to the forward light from 8 to the optical recording medium, it hardly transmits light. Can be avoided.

FIG. 6 shows the overall configuration of an optical head in which the hologram element 100 created by interference exposure is applied to the recording material containing the liquid crystal molecules described in FIGS.
In the figure, a hologram element 100 is arranged in parallel light between a collimating lens (for convenience, the reference numeral 10 shown in FIG. 15) and an objective lens (for convenience, the reference numeral 12 shown in FIG. 15). Yes.
Also in this case, by making the polarization direction of the incident light from the light source 8 to the hologram element 100 parallel to the track direction of the optical recording medium 13, the hologram element 100 has the ordinary light refractive index no of the entire liquid crystal part and the refractive index of the polymer part. Since the difference in np is not felt, the light is hardly diffracted and transmitted, so that light loss in the hologram element can be prevented.

  Next, an embodiment according to the second aspect of the present invention will be described.

FIG. 7 shows a plan view of the hologram area on the left side, and shows a divided detection area pattern of the diffracted light from the hologram and a photodetector for receiving it on the right side.
In FIG. 7, the knife edge method is used as the focus signal detection method, and the return light reflected by the optical recording medium is incident on the hologram area from the front side of the drawing, and the hologram area A diffracted light for focus signal detection. Condenses on the dividing line of the areas PD1-1 and PD1-2 of the photodetector.
As for the diffracted light for detecting the tracking signal, the diffracted light from the hologram region B is condensed on the detection region PD2, and the diffracted light from the hologram region C is condensed on the detection region PD3. The focus signal F is
F = [PD1-1]-[PD1-2]
The tracking signal T is generated from T = PD2-PD3.

  In the embodiment according to the second aspect of the present invention, when the reflected return light from the optical recording medium is incident on the hologram element 100 produced by interference exposure on the recording material containing the liquid crystal molecules described in FIGS. The polarization direction is substantially parallel to the direction projected onto the hologram surface of the optical axis of the diffracted light for focus detection.

  As a result, the extraordinary light refractive index ne of the entire liquid crystal part and the refractive difference of the polymer part are generated to diffract strongly. Thus, by optimizing the film thickness of the recording material containing liquid crystal molecules, it is possible to diffract one of the + 1st order diffraction efficiency with a high efficiency of 80% or more.

Based on this feature, the inventor conducted an experiment on the + 1st order diffraction efficiency of the hologram, and obtained the result shown in FIG.
FIG. 14 is a diagram illustrating the state of the + 1st order diffraction efficiency of the hologram, in which the horizontal axis indicates the position in the hologram and the vertical axis indicates the + 1st order diffraction efficiency.

As is apparent from FIG. 14, the + 1st order diffraction efficiency of the conventional rectangular diffraction grating is + 1st order diffraction efficiency of the hologram element produced by interference exposure on the recording material containing the liquid crystal molecules in this embodiment with respect to the curve S2. As shown by the curve S1, a hologram element having a diffraction efficiency more than twice that of the conventional method is realized.
Further, the polarization direction of the return light from the optical recording medium (for convenience, see FIG. 6 where reference numeral 13 shown in FIG. 15 is used) is projected onto the hologram surface of the optical axis of the diffracted light for focus detection; Substantially parallel can be realized by arranging a quarter-wave plate between the hologram element and the objective lens.

FIG. 8 shows a modification regarding another focus detection method, which is applied to focus detection by the double beam size method.
In FIG. 8, the reflected return light from the optical recording medium is divided into two diffracted lights by the hologram element and diffracted. In the figure, the left half of the return light is diffracted as a beam that is collected in front of the photodetector, and the right half of the return light is diffracted as a beam that is collected behind the photodetector. That is, the left half is the front convergence and the right half is the rear convergence, and the distance between the front and rear convergence positions from the photodetector is the same.

FIG. 9 is a plan view of the hologram element 100 and the photodetector 9. The hologram is divided into a region D and a region E. From the region D, diffracted light that converges on the front side is generated, and the photodetector 9 The light enters the detection region 1. Diffracted light that converges to the rear side is generated from the region E and enters the detection region 2 of the photodetector 9. In FIG. 9, the optical axes of the diffracted light from the regions D and E are depicted as being shifted up and down in the figure, but in actuality, the diffracted light coincides in the up and down direction of the figure. In the case of the figure, the focus signal F is
F = (1 + 3 + 5) − (2 + 4 + 6),
The track signal T is
T = (1 + 2 + 3) − (4 + 5 + 6) is detected. Here, the number “1” indicating a parameter means an output from the divided PD 1 in the photodetector 9.

  In the embodiment according to the second aspect of the present invention, the reflected return light from the optical recording medium is incident on the hologram element produced by interference exposure on the recording material containing the liquid crystal molecules described in FIGS. The return light polarization direction is substantially parallel to the direction projected onto the hologram surface of the optical axis of the diffracted light for focus detection.

  As a result, the extraordinary light refractive index ne of the entire liquid crystal part and the refractive difference of the polymer part are generated to diffract strongly. By optimizing the film thickness of the recording material containing liquid crystal molecules, the + 1st order diffraction efficiency of one side can be diffracted with a high efficiency of 80% or more.

Next, an embodiment according to the third aspect of the present invention will be described.
As in the embodiment according to the first aspect of the present invention, the hologram element used in this embodiment is one of a polymerizable monomer or a prepolymer in a composition containing non-polymerizable liquid crystal molecules as shown in FIG. A polymerizable liquid crystal monomer is mixed as a part, and is enclosed between two transparent substrates as shown in the upper cross-sectional view in FIG.
As the polymerizable liquid crystal monomer, a monofunctional liquid crystal acrylate monomer, a liquid crystal methacrylate monomer, a bifunctional liquid crystal diacrylate monomer, a liquid crystal dimethacrylate monomer, or the like is used. These materials may have a methylene chain between the acryloyloxy group which is a functional group and the liquid crystal skeleton. When such a composition is subjected to two-beam interference exposure, the polymer is polymerized while a certain amount of liquid crystalline monomer skeleton is also taken into the polymer. At this time, as in FIG. 3, in the process in which the non-polymerizable liquid crystal molecules are driven out from the bright part to the dark part of the interference fringes, the molecular long axis of the polymerizable liquid crystal monomer is moved by interacting with the polymerizable liquid crystal monomer. It is considered that a force to orient in the direction works.
In this manner, the polymerizable liquid crystal monomer is fixed in the polymer in a relatively aligned state as shown in the cross-sectional view of the middle stage in FIG. 10 and the top view shown in the lower stage. In particular, the interface between the polymer layer and the non-polymerizable liquid crystal is considered to be fixed in a state in which the liquid crystal skeleton major axis is aligned in a direction substantially perpendicular to the surface of the polymer layer. Accordingly, the orientation of the liquid crystal molecules is improved as a whole non-polymerizable liquid crystal layer, and the birefringence in the liquid crystal layer is further improved, so that the polarization selectivity of the diffraction phenomenon is increased. In addition, the liquid crystal skeleton is immobilized on the surface of the polymer layer and the interaction with non-polymerizable liquid crystal molecules is strong, so that the liquid crystal phase can be maintained up to a relatively high temperature, and the operating temperature range of the polarization selective hologram element can be increased. Can be spread.

  When a hologram element produced by interference exposure on the hologram recording material mixed with the above polymerizable liquid crystal is applied to an optical head, the light is incident on the hologram element from the light source in the same manner as in the embodiment according to the first aspect of the invention. By making the polarization direction of the light parallel to the track direction of the optical recording medium, the hologram element does not feel the difference between the ordinary light refractive index no of the entire liquid crystal part and the refractive index np of the polymer part, so it hardly diffracts and transmits. There is no loss of light in the hologram element.

  As in the case of the embodiment according to the second aspect of the invention, the polarization direction when the return light reflected by the optical recording medium is incident on the hologram element is expressed by the hologram surface on the optical axis of the diffracted light for focus detection. By making it substantially parallel to the direction projected onto the liquid crystal, the extraordinary light refractive index ne of the entire liquid crystal part and the refractive difference of the polymer part are generated, and the diffraction is strongly diffracted. By optimizing the film thickness of the recording material containing liquid crystal molecules, the + 1st order diffraction efficiency of one side can be diffracted with a high efficiency of 80% or more.

Next, an embodiment according to the fourth aspect of the present invention will be described.
In this embodiment, as described with reference to FIGS. 5, 6, and 8, a semiconductor laser as a light source is intended for a configuration in which a hologram element 100 created by interference exposure on a recording material containing liquid crystal molecules is applied to an optical head. The active layer bonding direction of 8 (LD) is characterized by being orthogonal to the track direction (track groove direction) of the optical recording medium.

  This has a radiation characteristic in which the radiation angle of the LD emitted light beam in the active layer joining direction is narrower than the radiation angle in the direction perpendicular to the joining. When the active layer bonding direction of the semiconductor laser 8 (LD) is set to be perpendicular to the track direction, and the emitted light beam is converted into parallel light by the collimator lens and condensed on the optical recording medium by the objective lens 12, the focused spot is in the track direction. It becomes a small and elliptical spot that is large in the direction perpendicular to the track.

  In recent high-density optical discs, it is required to increase the recording and reproducing resolution of fine recording marks and reduce the jitter of the reproduced signal. For this reason, it is necessary that the focused spot diameter in the track direction is small on the surface of the optical recording medium. Therefore, it is desirable to set the active layer bonding direction of the LD perpendicular to the track direction.

  From the above, when the active layer bonding direction of the semiconductor laser 8 (LD) is set to a direction perpendicular to the track direction of the optical recording medium 13, the polarization direction of the emitted light from the semiconductor laser 8 (LD) is the active layer bonding direction. Therefore, the polarization direction is also perpendicular to the track direction. In this state, the polarization direction of light incident on the hologram element produced by interference exposure on the recording material containing the liquid crystal molecules described in the embodiment according to the first aspect of the invention is the track direction as described in FIG. There is a defect that they cannot be parallel.

Therefore, in the present embodiment, in order to eliminate the above-described problem, the semiconductor laser 8 (LD) is configured so that the bonding direction of the active layer is perpendicular to the track direction of the optical recording medium as shown in FIG. A half-wave plate 110 is disposed between the (LD) and the hologram element 100, and the semiconductor laser 8 (LD) emitted light is polarized by the half-wave plate 110 when the direction of polarization of the light emitted from the semiconductor laser 8 (LD) is in the paper plane of FIG. The direction is changed to be parallel to the track direction of the optical recording medium, and then incident on a hologram element formed by interference exposure on a recording material containing liquid crystal molecules.
This method makes it possible to achieve both the desired spot shape for recording and reproduction of a high-density optical disc and the forward non-diffracting and total transmission characteristics in a hologram element produced by interference exposure on a recording material containing liquid crystal molecules.

Next, an embodiment according to the fifth aspect of the present invention will be described.
FIG. 12 shows a partial modification of the configuration shown in FIG.
Here, a semiconductor laser 8 (LD) as a light source and a recording material containing liquid crystal molecules and a half-wave plate 110 arranged between the hologram element 100 created by interference exposure are integrated with the hologram element 100.

As an integration method, a method of attaching an inorganic wavelength plate such as quartz to a light source side transparent substrate of a hologram element, or a method of using the transparent substrate on the light source side of the hologram element itself as an inorganic half-wave plate 110, As another method, a half-wave plate 110 using an organic stretched film is attached to the light source side transparent substrate of the hologram element 100, or an inorganic substance such as Ta ₂ O _{5 is formed} on the light source side transparent substrate. This can be realized by the half-wave plate 110 formed by oblique vapor deposition.

Next, an embodiment according to the sixth aspect of the present invention will be described.
In the optical head device having the configuration shown in FIGS. 5, 6, 8, 11 and 12, as shown in FIG. 13, the recording material including the light source 8 (LD), the photodetector 9, and liquid crystal molecules is subjected to interference exposure. By constructing the created hologram element 100 in one unit, the assembly time is reduced because the light source 8, the photodetector 9, and the detection optical system are integrated when the optical head device is assembled. And the adjustment becomes easy. This integrated configuration includes integration of the half-wave plates shown in FIGS. 11 and 12 as indicated by reference numeral 110.

It is a cross-sectional schematic diagram which shows the structure of the hologram element used for the Example which concerns on invention of Claim 1. It is a figure which shows the hologram formation process by the phase separation at the time of two-beam interference exposure execution for the hologram element shown in FIG. It is a figure which shows the state in the bright part and dark part in the interference fringe in the formation process shown in FIG. It is a figure which shows the orientation state in the liquid-crystal part in the last process of a hologram formation process. FIG. 5 is a diagram illustrating a case where a hologram element including the liquid crystal unit illustrated in FIGS. 2 to 4 is provided in an optical head. It is a figure which shows the whole structure of the optical head apparatus provided with the optical head shown in FIG. FIG. 5 is a diagram showing a focus control state when a hologram element including the liquid crystal unit shown in FIGS. 2 to 4 is used as an embodiment according to the second aspect of the invention. It is a figure which shows the modification regarding the focus control shown in FIG. FIG. 9 is a layout diagram of hologram elements and photodetectors used in a modification of focus control shown in FIG. 8. It is a figure which shows the state change in the liquid-crystal part by the structure of the hologram element used for the Example which concerns on invention of Claim 3, and phase separation at the time of two-beam interference exposure. It is a figure for demonstrating the structure of the optical head apparatus by the Example based on invention of Claim 4. It is a figure for demonstrating the structure of the optical head apparatus by the Example based on invention of Claim 5. FIG. It is a figure for demonstrating the structure of the optical head by the Example which concerns on invention of Claim 6. FIG. It is a diagram for demonstrating the diffraction efficiency by the Example which concerns on invention of Claim 2 shown in FIG. It is a figure for demonstrating the conventional structure of an optical head apparatus. It is a figure which shows the structure of the polarization | polarized-light diffraction element used for the optical head apparatus shown in FIG. It is a diagram for explaining the diffraction efficiency characteristics of the + 1st order diffracted light (one side diffracted light) with respect to the incident angle of the polarizing diffraction grating.

Explanation of symbols

DESCRIPTION OF SYMBOLS 8 Semiconductor laser 9 Photodetector 10 Collimating lens 12 Objective lens 13 Recording material 100 Polarization selective hologram element 101,101 'Transparent substrate 102 Nonpolymerizable liquid crystal 103 Polymerizable monomer 110 1/2 wavelength plate

Claims (6)

In an optical head device comprising an optical head that focuses light from a light source on a recording medium and detects reflected light from the recording medium to record or reproduce information, or record and reproduce information.
As a hologram element used for the optical head, at least a composition comprising a non-polymerizable liquid crystal having dielectric anisotropy, a polymerizable monomer or prepolymer, and a photopolymerization initiator is held between a pair of transparent substrates,
By subjecting the composition to two-beam interference exposure, a polymer-dispersed liquid crystal type polarization selective hologram element in which a periodic phase separation structure of a layer mainly composed of a polymer and a layer mainly composed of a non-polymerizable liquid crystal is formed. And an optical head device characterized in that a polarization direction of light incident on the hologram element from a light source (electric field vibration direction) is set substantially parallel to a recording medium track direction. The optical head device according to claim 1,
As the polarization direction of the return light reflected by the recording medium and incident on the hologram element (vibration direction of the electric field), it is substantially parallel to the direction in which the optical axis of the diffracted light for focus detection diffracted by the hologram is projected onto the hologram surface. An optical head device, characterized in that The optical head device according to claim 1 or 2,
The hologram element uses a polarization-selective hologram element that further contains a polymerizable liquid crystal monomer in the composition constituting the hologram and contains a liquid crystal skeleton in a layer mainly composed of a polymer after two-beam interference exposure. An optical head device. The optical head device according to any one of claims 1 to 3,
A polymer-dispersed liquid crystal polarization selective hologram element is used as the hologram element, a semiconductor laser is used as a light source, and the bonding direction of the semiconductor laser (the bonding direction of the active layer) is set to be perpendicular to the track direction of the recording medium. An optical head device comprising a half-wave plate disposed between the light source and the hologram element. The optical head according to claim 4, wherein
The optical head device, wherein the half-wave plate is integrated with a hologram element. The optical head device according to any one of claims 1 to 5,
An optical head device, wherein the light source, the photodetector, and the hologram element are integrated.

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